Present methods of grading structural lumber for strength include visual grading and machine grading. At best these methods only explain about 50 percent of the variability in strength because they do not entirely account for the heterogeneous nature of lumber. A piece of structural lumber is an assemblege of various lumber characteristics which range from clear, dense, straight-grained wood with high strength to knotty, low-density, cross-grained wood with low strength. The purpose of any grading method would be to account for these variations and assign a stress rating (i.e., a predicted strength level) to each piece. Visual stress rating is based primarily on an empirical estimation of the size of knots in each piece as they are limited by a specific grading rule. Inherent errors in judgment due to the physical demands on the visual grader lead to inaccurate assessments of lumber strength. More accurate estimates of the low-strength, low-density areas around knots are needed to improve upon this visual stress-rating method.
The machine stress-rating methods are an improvement over the visual approach, but also have some significant limitations. The most common machine stress-rating technique physically bends the lumber and measures its stiffness. Stress ratings, or predicted strength levels, are assigned to each based on established strength-to-stiffness relationships. This physical bending of the lumber, along with established visual limits (overrides) on knot sizes, leads to the final grade of the piece. A problem with this method is the large span required to adequately bend the piece and the inherent insensitivity of such a long span system to the short, low-strength, low-density, cross-grain areas often found in lumber. Therefore, this machine grading approach to grading structural lumber needs a more accurate estimate of the short, low-strength areas around knots before improved strength estimates are possible. The solution to this problem is specifically addressed by this invention.
Another, but less common, machine grading technique for structural lumber creates an energy wave within the piece and monitors the speed of the wave. A dynamic modulus of elasticity (MOE) is calculated as the product of wave speed and density. Taken over the full length of a lumber piece, an average MOE can be obtained from the wave speed and the density can be obtained from the size and weight of the specimen. More accuracy and a more desirable measurement of MOE would come about if the speed of the wave was obtained for short spans or short segments of the piece. This procedure would provide localized strength information for more accurate strength estimates. The problem that remains is to nondestructively obtain the short segment density needed for each short segment MOE calculation. The solution to this problem is specifically addressed by this invention.
Another important practical application of this invention is directed to an inherent problem associated with the waterborne preservative treatment of wood. Waterborne preservative treatments have been found to reduce the strength of many types of treated wood products. In lumber (nonclear specimens), strength reductions possibly as high as 25 percent are mentioned in ASTM D 245 (American Society for Testing and Materials 1978). This strength loss seems to be caused by both the hydrolytic chemicals and the temperature sustained in the subsequent kiln-drying process. It is suspected that kiln drying of waterborne preservative-treated lumber may significantly reduce its strength in localized areas. This reduction in strength is believed to be directly related to a reduction in density. Therefore, the solution, as stated previously, is to nondestructively obtain the short segment density. This is specifically addressed by this invention.